In the auto industry, demand has been rising for steel sheet provided with the properties of both shapeability and high strength so as to achieve both lighter weight of the chassis to deal with environmental problems and safety in collisions.
To deal with these needs, Japanese Unexamined Patent Publication (Kokai) No. 5-59429 discloses steel sheet having as the steel sheet structure a mixture of the three phases of the ferrite phase, bainite phase, and austenite phase and transforming the residual austenite to martensite at the time of shaping so as to utilize the transformation-induced plasticity exhibiting a high ductility. This type of steel sheet for example forms a complex structure by the addition, by wt %, of C: 0.05 to 0.4%, Si: 0.2 to 3.0% A, and Mn: 0.1 to 2.5% in the steel and controlling the temperature pattern in the process of annealing in the two-phase region, then cooling and is characterized in that the desired properties can be brought out without the use of expensive alloy elements.
When zinc plating this steel sheet by a continuous molten zinc plating system, usually the surface of the steel sheet is degreased, the surface is cleaned, then, for the purpose of forming the above-mentioned structure, the sheet is heated in an nonoxidizing furnace to form an iron oxide layer of a thickness of 50 nm to 1 μm or so on the surface of the steel sheet, annealing the sheet in a reducing furnace to reduce the iron oxide layer, then dipping the sheet in a molten zinc plating bath to plate it with zinc. When producing an alloyed molten zinc plated steel sheet, the steel sheet is dipped in a plating bath in that step, then held at a temperature of 400 to 600° C. or so to alloy the zinc and iron and convert the plating layer to an alloy phase of Fe and Zn constituting an δ1 phase.
Steel sheet, however, contains large amounts of easily oxidizing elements such as Si and Mn compared with the ordinary deep drawn cold-rolled steel sheet etc., so there is the problem that the surface of the steel sheet is easily formed with Si oxides, Mn oxides, or Si and Mn complex oxides in the heat treatment performed in the above series of steps. However, in industrial scale systems, it is difficult to reduce the oxygen potential of the atmosphere in the heating step to an extent where Si or Mn will not be oxidized, so formation of Si and Mn oxides at the surface of the steel sheet is substantially unavoidable. Further, if the surface of the steel sheet is formed with an Si oxide layer or Mn oxide layer, there is the problem that the alloying of the Zn and Fe is inhibited in the alloying step at the time of production of the alloyed molten zinc plated steel sheet and parts where the Fe—Zn alloy phase have not yet been formed remain.
One method easily conceivable as a means for solving these problems is to set the alloying treatment temperature slightly high to promote alloying of Fe and Zn. At the alloying treatment temperature of 450 to 600° C., however, austenitic transformation occurs in the steel sheet, so if setting the alloying treatment temperature slightly high, depending on the holding time, the structure of the steel sheet will not become the desired mixed structure of a mixture of the three phases of the ferrite phase, bainite phase, and austenite phase. As a result, there is the problem that the shapeability and strength of the steel sheet aimed at cannot be secured in some cases.
To deal with this problem, Japanese Unexamined Patent Publication (Kokai) No. 55-122865 discloses the method of forming a 40 to 1000 nm iron oxide layer on the surface of a steel sheet in a heat treatment step by a nonoxidizing furnace in a continuous molten zinc plating step so as to prevent outward diffusion of the Si or Mn in the reduction step, suppress the formation of the Si oxide layer, and improve the plating properties. With this method, however, if the reduction time is too long for the thickness of the iron oxide layer, Si will become dense at the surface of the steel sheet and an Si oxide layer will be formed, while if the reduction time is too short, iron oxide will remain on the surface of the steel sheet and defects in the plating properties, that is, the formation of unformed parts of the Fe—Zn alloy phase will be formed. Further, in recent continuous molten zinc plating systems, annealing systems using radiant type heating furnaces rather than nonoxidizing furnaces are becoming the mainstream. In such systems, there was the problem that the above method could not be used.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2000-309824 discloses as a method for preventing selective oxidation of the Si or Mn at the time of annealing the method of hot rolling the steel sheet, then heat treating it in the state with the black skin scale still attached in an atmosphere where reduction will substantially not occur and in a temperature range of 650 to 950° C. so as to form a sufficient internal oxide layer in the base iron surface layer. With this method, however, in addition to the conventional continuous molten zinc plating step, a heat treatment step for forming the internal oxide layer and a pickling treatment step become necessary, so there was the problem that a rise in production costs was invited. Further, the plated steel sheet having the internal oxide layer had the problem of easily peeling of the plating layer.